This invention relates to a driving wheel control system which suppresses excessive slip of driving wheels of an automotive vehicle, and more particularly to a driving wheel control system of this kind which is adapted to reduce the output of an engine installed in the vehicle by controlling the opening degree of a throttle valve of the engine to thereby suppress excessive slip of the driving wheels.
A throttle valve of an internal combustion engine installed in an automotive vehicle is usually operated by means of an accelerator pedal. A driving wheel control system is known, which is called "traction control system" (hereinafter referred to as "TCS"). This system is operable upon slippage of a driving wheel or driving wheels depending upon road conditions to drive the throttle valve toward a smaller opening degree by means of a pulse motor, independently of the position of the accelerator pedal, to thereby temporarily reduce the engine output and hence promptly eliminate slip of the driving wheel(s).
On the other hand, a pulse motor in general is controlled in open loop mode such that its position is determined by the number of stepping pulses generated in accordance with the difference between a predetermined reference position (initial position) and a desired position, as disclosed in Japanese Provisional Patent Publication (Kokai) No. 62-821995. However, before the reference position is determined or initialized, i.e. at the time of resetting or initialization of a computer which controls the pulse motor, the actual position of the pulse motor is unknown. Therefore, on such an occasion the pulse motor has to be controlled in feedback mode in response to the difference between the actual position of the pulse motor sensed by a motor position sensor (throttle valve opening sensor) and a desired position so that the former becomes equal to the latter, as disclosed e.g. in Japanese Provisional Patent Publication (Kokai) No. 61-94597.
In the case where the TCS carries out the throttle valve control by the use of a pulse motor in feedback mode as mentioned above with an actual opening degree (hereinafter abbreviated as "opening") of the throttle valve detected by a throttle valve opening sensor being a reference value of the motor position, the feedback control has degraded control accuracy due to output characteristic errors attributed to non-linearity in the sensor output, hysteresis, dead zone, etc.
Therefore, once the reference position of the pulse motor has been determined, the throttle valve control of the TCS by the use of a pulse motor should be carried out in open loop mode in order to achieve high control accuracy.
To carry out the open loop control with high accuracy, it is essential to accurately determine the reference position of the pulse motor. However, when the TCS is at rest, the pulse motor is kept inoperative in a waiting position corresponding to a fully open position of the throttle valve, whereas upon the start of the TCS it is driven in the direction of moving the throttle valve toward its closed position. That is, the motor position and the throttle valve opening do not always coincide with each other. As a result, if the detected opening value of the throttle valve opening sensor is stored as a reference position of the pulse motor in order to carry out the open loop control when the positional relationship between the motor and the throttle valve is still uncertain such as at or immediately after the start of the TCS, the stored reference position can be incorrect, resulting in the throttle valve being controlled into an improper position.
Therefore, the reference position of the pulse motor should not be determined until after a predetermined initialization condition is satisfied when the positional relationship between the pulse motor and the throttle valve has become stable after the pulse motor was driven from its waiting position upon the start of operation of the TCS.
On the other hand, in the case where the throttle valve control by a pulse motor is carried out in feedback mode before the determination of the reference position of the pulse motor as mentioned above, there arises the following problem:
When the throttle system (the pulse motor, the throttle valve and its peripheral elements) is normally functioning, the pulse motor ought to be driven into a position corresponding to the desired position of the throttle valve from the waiting position corresponding to the fully open position of the throttle valve, within a predetermined time of period (e.g. 1 sec.) from the time a TCS operating condition (a condition under which the pulse motor needs to be controlled to a closed position) becomes satisfied.
However, in the event of occurrence of any of the following phenomena, the pulse motor cannot be properly driven, so that the aforementioned initialization condition remains unsatisfied even after the lapse of the above predetermined period of time after the fulfillment of the TCS operating condition:
i) Skipping of the pulse motor due to shortage of motor torque caused by a drop in a supply voltage from the battery;
ii) Biting of the throttle valve and bending of the link;
iii) Temporary failure of the throttle valve or the like.
If in such a case as above fulfillment of the initialization condition is awaited, the feedback control is continued based upon incorrect detected opening of the throttle valve due to failure of the throttle system, whereby the pulse motor cannot be moved to the desired position, resulting in failure to properly reduce the engine output and hence eliminate slip of the driving wheel(s).
Upon the start of the TCS, the pulse motor is driven from a waiting position corresponding to the fully open position of the throttle valve and in the direction of closing the throttle valve, while during operation of the TCS the throttle valve is moved to a desired opening .theta..sub.THO in the vicinity of its fully closed position and held thereat. When slip of the driving wheel(s) has been dissipated, the TCS is rendered inoperative and then the pulse motor is again moved back to the waiting position corresponding to the fully open position of the throttle valve and then deenergized to be held thereat.
However, in the case where the operating time of the TCS is so short that the TCS is rendered inoperative before the initial or reference position of the pulse motor is determined, if the pulse motor is driven to open the throttle valve by the feedback control in such a case, the output from the throttle valve opening sensor does not show a larger opening value than the accelerator pedal angle even when the pulse motor is supplied with a driving signal for opening the throttle valve so that it is driven into the waiting position corresponding to the fully open position of the throttle valve. Since in the feedback control the output from the throttle valve opening sensor is regarded as the pulse motor position, it cannot be determined that the pulse motor has been driven into the waiting position corresponding to the fully open position of the throttle valve. As a result, there occurs an inconvenience that the above driving signal continues to be supplied to the pulse motor.